If you've already upgraded to ZCS 5.0.7 and have no concern for any of the bugs listed here, then there's no pressing need to update at this time. Users running versions earlier than 5.0.7 should upgrade directly to 5.0.8.

iPhone3G GA support is currently planned for ZCS 5.0.9, and this release does not add any additional functionality for iPhone above it's beta support in 5.0.7. You can read more about the new Zimbra Mobile integration here: Zimbra Mobile for iPhone » Zimbra :: Blog